Introduce asdf.util.load_yaml

[braingram]

Description

This PR introduces asdf.util.load_yaml to load the yaml contents of an ASDF file.

import asdf

# load a tree containing, lists, dicts and strings (no tags)
tree = asdf.util.load_yaml("test.asdf")

# load a tree containing asdf.tagged.Tagged instances
tagged_tree = asdf.util.load_yaml("test.asdf", tagged=True)

This is an alternative to #1677 and can be used to replace the (many) uses of _force_raw_types in downstream code and _get_yaml_content (see note below about sphinx-asdf).

It should be possible to remove _force_raw_types once all the downstream packages have removed its usage. This PR removes the use of this argument in asdftool edit. asdftool diff will require more work to remove the use of _force_raw_types. At the moment this PR does not deprecate nor remove _force_raw_types.

Although _get_yaml_content is no longer used in asdf with this PR it is used in sphinx-asdf and will require a release and fix (see asdf-format/sphinx-asdf#87) and release of sphinx-asdf before removal.

Checklist:

• pre-commit checks ran successfully
• tests ran successfully
• for a public change, a changelog entry was added
• for a public change, documentation was updated
• for any new features, unit tests were added

[eslavich]

I like this, just the one question about BaseLoader.

[eslavich]

What does BaseLoader do when it encounters a tag that it doesn't know how to handle? I thought I remembered that it throws an error in that case but clearly not...

[braingram]

Thanks for taking a look!

My understanding is the BaseLoader ignores all tags. The pyyaml documentation has a rather brief description:

BaseLoader(stream) does not resolve or support any tags and construct only basic Python objects: lists, dictionaries and Unicode strings.

Throwing an empty ASDF file at it:

#ASDF 1.0.0
#ASDF_STANDARD 1.5.0
%YAML 1.1
%TAG ! tag:stsci.edu:asdf/
--- !core/asdf-1.1.0
asdf_library: !core/software-1.0.0 {author: The ASDF Developers, homepage: 'http://github.com/asdf-format/asdf',
  name: asdf, version: 3.0.2.dev46+gfc646c3c}
history:
  extensions:
  - !core/extension_metadata-1.0.0
    extension_class: asdf.extension._manifest.ManifestExtension
    extension_uri: asdf://asdf-format.org/core/extensions/core-1.5.0
    software: !core/software-1.0.0 {name: asdf, version: 3.0.2.dev46+gfc646c3c}
...

with:

d = yaml.load(open('foo.asdf').read(), Loader=yaml.BaseLoader)

yields:

> d
{'asdf_library': {'author': 'The ASDF Developers',
  'homepage': 'http://github.com/asdf-format/asdf',
  'name': 'asdf',
  'version': '3.0.2.dev46+gfc646c3c'},
 'history': {'extensions': [{'extension_class': 'asdf.extension._manifest.ManifestExtension',
    'extension_uri': 'asdf://asdf-format.org/core/extensions/core-1.5.0',
    'software': {'name': 'asdf', 'version': '3.0.2.dev46+gfc646c3c'}}]}}

inspecting the extension item:

> type(d['history']['extensions'][0])
dict

In the pyyaml source BaseLoader uses BaseConstructor for building objects and BaseConstructor is the parent class for SafeConstructor.